In general, line pipes are produced by forming a steel plate produced in a plate mill or a hot-rolling mill, by UOE forming process, press bend forming process, roll forming or the like. Line pipes for use for transportation of hydrogen sulfide-containing crude oil or natural gas (hereinafter this may be referred to as “line pipes for sour gas service”) are required to satisfy so-called “sour resistance” such as resistance to hydrogen induced cracking (HIC resistance), resistance to anti-stress corrosion cracking (SCC resistance) and the like, in addition to strength, toughness and weldability. Hydrogen induced cracking (hereinafter referred to as HIC) of steel is said as follows: Hydrogen ions from corrosion reaction adhere to the surface of steel and permeate into the inside of steel as atomic hydrogens, then diffuse and accumulate around the non-metal inclusions such as MnS and the like or hard second phase in steel and then form hydrogen gas thereby cracking the steel owing to the inner pressure thereof.
Heretofore, for preventing such hydrogen induced cracking, some methods have been proposed. For example, JP-A 54-110119 proposes a technique of reducing the S content of steel and adding a suitable amount of Ca, REM (rare-earth metal) or the like to steel to thereby prevent the formation of long-extending MnS and convert the shape into a finely dispersed spherical CaS inclusion. Accordingly, the stress concentration by the sulfide inclusion is reduced and cracking is therefore prevented from initiation and propagation to thereby improve the HIC resistance of steel.
JP-A 61-60866 and JP-A 61-165207 propose a technique of reducing center segregation through reduction in elements having a high tendency toward segregation (C, Mn, P, etc.) or through soaking heat treatment in a slab heating process, and changing the microstructure of steel in to bainite phase by accelerated cooling after hot rolling. Accordingly, formation of an island martensite (M-A constituent) to be a initiation point of cracking in the center segregation area, as well as formation of a hardened structure such as martensite or the like to be a propagation path of cracking can be prevented. JP-A 5-255747 proposes a carbon equivalent formula based on a segregation coefficient, and proposes a method of preventing cracking in the center segregation area by controlling it to a predetermined level or less.
Further, as countermeasures to the cracking in the center segregation area, JP-A 2002-363689 proposes a method of defining the segregation degree of Nb and Mn in the center segregation area to be not over a predetermined level, and JP-A 2006-63351 proposes a method of defining the size of the inclusion to be the initiation point of HIC and the hardness of the center segregation area.
However, heavy wall pipes having a wall thickness of at least 20 mm are increasing for recent line pipes for sour gas service; and in such heavy wall pipes, the amount of alloying elements to be added must be increased for securing the strength thereof. In that case, even when the MnS formation is prevented or the microstructure of the center segregation area is improved according to the above-mentioned prior-art methods, the hardness of the center segregation area may increase and HIC may occur from Nb carbonitride. Cracking from Nb carbonitride has a small crack length ratio, and therefore it has heretofore not been specially taken as a problem in the conventional requirement for HIC resistance. However, recently, further higher HIC resistance is required, and it has become necessary to prevent HIC from Nb carbonitride.
The method of reducing the size of an Nb-containing carbonitride to an extremely small size of 5 μm or smaller, as in JP-A 2006-63351, may be effective for preventing the occurrence of HIC in the center segregation area. In fact, however, coarse Nb carbonitride may often form in the finally-solidified zone in ingot casting or continuous casting; and for the above-mentioned severer request for HIC resistance, the material of the center segregation zone must be extremely strictly controlled for preventing initiation of HIC and for preventing the propagation of cracking from the Nb carbonitride that may form at some frequency. As the method of controlling the material of the center segregation area, there is mentioned the carbon equivalent formula proposed by JP-A 5-255747 in which a segregation coefficient is taken into consideration. However, since the segregation coefficient is experimentally obtained through analysis with an electron probe micro analyzer, it can be obtained only as a mean value within the measurement range of the spot size of, for example, around 10 μm or so. Also, this is not a method capable of strictly estimating the concentration of the center segregation area.
Accordingly, it could be helpful to provide a steel plate for high-strength line pipes excellent in HIC resistance, in particular, a steel plate for high-strength line pipes for sour gas service that has excellent HIC resistance capable of sufficiently satisfying the severe requirement for HIC resistance necessary for line pipes for sour gas service having a pipe thickness of 20 mm or more.
It could also be helpful to provide a steel pipe for line pipes, which is formed of the high-strength steel plate for line pipes having such excellent capabilities.